1. Field of the Invention
The present invention relates to motor-driven let-off and take-up technology for looms, and more particularly to a method of and an apparatus for feedforward control effective for transient operation at the time of starting a loom.
2. Description of the Prior Art
Let-off motions for looms serve to feed a warp yarn from a supply beam under suitable tension, and take-up motions serve to progressively wind a woven fabric under given tension. These motions are controlled by a control system which is a tension control system for controlling the warp yarn of the woven fabric with the warp yarn tension as a control target. As disclosed in U.S. Pat. No. 3,802,467 (DE-AS No. 2 206 781), U.S. Pat. No. 4,031,923 (DE-AS No. 2 555 986), and U.S. Pat. No. 4,407,331 (DE-AS No. 2 939 607), for example, a typical tension control system is composed of a main loop or a tension control system for effecting PI (proportional-plus-integral) control and a minor loop or a speed control system having an increased response speed.
The tension of a warp yarn pulsates during one revolution in response to principal movement of the loom. The pulsating tension variation is normally not controlled by the tension control system. The general tension control system of the type described has an integrating circuit having a large time constant as disclosed in the foregoing prior patents. Any tension variations in one revolution of the loom are absorbed by an integrating function of the integrating circuit. Because of the presence of the integrating function, however, the detection by the tension control system of any tension variations is slow, and the loom is subjected to a large operational variation when it is started again.
The loom generally has a weft stop motion for automatically stopping the loom at the time of a weft insertion failure. When the loom is stopped, the weft yarn is restored. If the operation of the loom is interrupted, then the fabric being woven will have a stop mark corresponding to the position of the interruption, the stop mark being a product flaw. The stop mark is produced since (1) the fell is retracted by extracting the defective weft yarn upon a weft insertion failure, (2) the yarns are elongated and the fabric shrinks while the loom stops for a long time, and (3) the inserted weft yarn is beaten up under an insufficient force at the time the loom is started.
While the loom is in a transient operation, therefore, the tension control system is incapable of ideal control of warp tension due to the integrating function thereof with the large time constant. To prevent stop marks from being produced, control would be effected for optimizing the rate of letting off the warp yarn when the loom is in a transient mode such as inching operation, reverse operation, and starting operation. However, such a control mode would retard the response of the tension control system, failing to achieve reliable control. Accordingly, the conventional tension control system is incapable of optimum control especially in a transient loom operation such as an initial starting operation or a restarting operation, and cannot avoid stop marks produced in woven fabrics.